1. Field of the Invention
The present invention relates generally to power control apparatus in AC power systems and more specifically, to a programmable power control apparatus adapted for sensing changes in a load such as a bank of fluorescent lights for appropriately changing the voltage applied to the load.
2. Prior Art
The present invention is particularly suited for operation with an apparatus for control of load power consumption of the type described in co-pending patent application Ser. No. 406,410 filing date 8/9/82 now U.S. Pat. No. 4,431,948 issued 2/14/84 and commonly owned by the assignee hereof. That load power consumption control apparatus permits the application of a normal high voltage to a lighting load such as a bank of fluorescent lights and permits selective reduction of the voltage to maintain such lights in a lighting condition but with a reduced power consumption. Typically, the voltage reduction is maintained for a selected period of time and then the voltage is increased automatically for a selective period of time. This is one way of assuring that if any additional lights have been turned on during the low voltage condition, that some time shortly thereafter, a sufficiently high voltage will be applied to ignite the newly added fluorescent light load before the voltage is again reduced. One such periodic power increasing apparatus for use in a similar type of power control apparatus is disclosed in U.S. Pat. No. 4,189,664 to Hirschfeld. In the Hirschfeld apparatus the output voltage is maintained at a reduced level for a majority of the time except that periodically the output voltage is returned to a high level for short periods of time. Each time the output is returned to a high level any fluorescent load on the line can be switched on. Once the fluorescent load is switched on, the voltage is again reduced. Accordingly, this is one prior art manner in which fluorescent loads are operated at a reduced voltage while still permitting fluorescent loads to be turned on at frequent intervals when the output level is returned to a high level. Unfortunately, this scheme for periodic increase in the otherwise reduced power level suffers from a number of disadvantages. For example, in such a prior art apparatus the voltage is increased periodically irrespective of whether additional lights have been turned on. As a result, the average power savings is needlessly diminished, particularly in those systems where additional lighting loads are very infrequently added to the lighting system. Another exemplary disadvantage of such a prior art apparatus stems from the fact that a considerable amount of time may have to elapse before the high voltage is applied to the load in order to start the newly added fluorescent lighting load. Thus for example, if additional lights have been switched on immediately after a previously high voltage condition has been returned to a low voltage condition, a sufficient voltage to start the newly added lights will not be applied until the subsequent voltage increase and as a result the user will have to wait what may be an inconveniently long period of time before the lights are actually ignited and fully on. Thus in such prior art periodic voltage increase systems there is an inherent trade off between the desire to minimize load power consumption as a result of reducing voltage to the load and the desire to provide the users with a minimum of inconvenience in regard to waiting for newly switched on lights to come on fully and provide complete lighting.